The overall objectives of the research project are: 1) To detect early focal changes in mitotic activity of rat mammary epithelium associated developmentally with the appearance of hyperplastic alveolar nodules (HANs); 2) To solate tissues with such aberrant mitotic activity from the mammary gland of untreated and carcinogen-treated rats and test by transplantation techniques the hypothesis that they are developmental precursors of HANs; and 3) To study the biological properties (morphologic, neoplastic and cell kinetic) of the presumptive HAN precursor tissues to ascertain how differ normal and neoplastic mammary tissue. In the first two years of support, we have characterized by means of a histotopological technique the mitotic activity of mammary gland epithelium in normal rats (aged 30-215 days) and in carcinogen (DMBA)-treated rats at different times after exposure to carcinogen. These studies have shown that an alteration in the pattern of mammary gland mitotic activity develops in untreated and carcinogen-treated rats prior to the appearance of HANs and mammary adenocarcinomas. The establishment of an altered pattern of mitotic activity in the mammary gland prior to the development of hyperplastic and neoplastic lesions suggests a relationship of the altered replicative pattern to the subsequent development of HANs and mammary tumors. Presumptive HAN precursor tissue will now be isolated form the mammary glands of untreated and carcinogen-treated rats and established as in vivo lines for detailed cellular kinetic study. The mitotic activity, morphology, incidence of HANs and tumors in the transplant lines, as well as certain other properties of these lines will be correlated with the relative mitotic activity of the tissue used to establish the transplant lines. These parameters will also be correlated with the estrous cycle status of the donor rats. In this way we will attempt to prove that morphologically normal mammary tissues with the characteristics of abnormally high and out-of-phase mitotic activity, relative to the normal pattern of mammary epithelial cell proliferation during the estrous cycle, are developmental precursors of HANs and/or mammary adenocarcinomas.